Methods using bacterial plasmids have permitted obtaining strains of bacteria producing peptides and proteins, such as somatostatine, insulin and ovalbumin on an industrial scale. However, these bacterial systems are subject to limitations due to the differences which exist in the systems of transcription and of translation between eucaryotes and procaryotes. Moreover, a number of eucaryotes genes are discontinuous and have insertions (up to 7 for chicken ovalbumin). Consequently, such genes cannot be properly translated by a procaryote, unless a gene without insertions is constructed.
Yeasts which are eucaryote microorganisms offer the possibility of removing these constraints, and thus have great potential interest for many practical applications.
However, until recently, it was not known how to effect the entry of an exogenous deoxribonucleic acid (DNA) into yeast, cause it to remain there in stable form and express itself. The work of A. Hinnen, J. B. Hicks and G. R. Finck, Proc. Natl. Acad. Sci. USA (1978), 75, 1929-33, discloses that by using a bacterial plasmid vector bearing the gene LEU.sub.2.sup.+ of yeast, clones of yeasts are obtained that have integrated all or part of the vector without, however, the latter being maintained autonomously in the cytoplasm. This method has the disadvantage, however, of not permitting the amplification of the original exogenous DNA.
It is known, that certain bacterial plasmids exist in multiple-copy state in the bacteria which bear them and it is also known that there exists in certain yeast strains, particularly Saccharomyces cerevisiae, a plasmid designated as 2.mu. because of its length. This 2.mu. plasmid exists at the level of about 50 to 100 copies per cell and while its precise genetic function is not known, it is known that it can be transcribed, at least in part, and, consequently, can constitute a vector of potentially great interest.